Animal Instincts
by EnchiladaDan
Summary: AU where everyone - male and female alike - is either an alpha, a beta, or omega. The Winchesters are no exception to this rule and, surprisingly, neither is their friendly neighborhood angel, Castiel. One of the boys is an omega in heat and needs an alpha to calm him. He asks for 1, but gets 2 alphas. Pairings: Sam/Dean, and Dean/Castiel. Explicit content.


**A/N: Contains sibling incest, underage sex / underage incest. Please do not read if this bothers you.**

Dean felt a fever creeping up the back of his neck. His cheap polyester suit felt like it was sticking to him, and his hands sweated against the leather of his steering wheel. Without a word, he turned the impala around.

"Dean, what are you doing?"

"Sammy, we gotta go back to the hotel."

"What, why? Dean, we have _a whole day_ of questioning witnesses ahead of us."

Dean shifted uncomfortably in his seat and amended, "Well… I have to. Sam, you have to do this solo."

He looked over at Sam momentarily and Sam sniffed the air. It _couldn't_ be…

"Dean… is that what I think it is?"

"It's been a month since Tulsa, Sam. I can't be in public." He said it irritably, but Sam was unruffled. He knew Dean couldn't really control his temper when he was like this.

Sam sniffed the air again, the impala's air becoming stifling. "But shouldn't that reset it?.."

"No, Sammy, it doesn't work like that. I fucked up my suppressants in Tulsa. But this was _bound_ to happen, Tulsa or no."

"So it's been 6 months huh?"

Dean nodded tersely, and peeled into the hotel parking lot. He practically shot up from the seat and bolted into the room. Sam followed quickly after, catching the keys Dean threw at him before he began to peel off his suit jacket. _"It's so goddamn hot,"_ he griped as he struggled with his tie. Sam pocketed the keys, walked slowly over to his brother, and took the tie from his shaking fingers. Dean let Sam slide the tie off of him as Dean worked his cuff buttons loose.

"Sammy, you gotta _leave_. You have to question those people…" There were no convictions behind his words and he didn't pull away when Sam put the back of his hand against Dean's forehead.

"You're burning up," he stated, and Dean just nodded in response, his fingers working the buttons of his dress shirt.

He peeled it off, revealing a white undershirt. Sam moved towards the bathroom, and came back with a bottle of Tylenol and a pitcher of water. He set them on Dean's nightstand and said, "I'm going for ice. It's worse than usual." He picked up both ice buckets and left the room.

Dean sat down on his bed, his hands still shaking as he kicked off his dress shoes and peeled off his socks. He took a deep breath and reached for a water glass, and the bottle of Tylenol. He dropped the empty glass on the comforter as he worked on opening the pill bottle. He struggled with the ridiculous cap and finally managed to work it off by the time Sam returned. God damn childproof caps.

Sam placed the ice buckets on his nightstand and went to retrieve the pitcher. By the time he poured Dean a glass of ice water, Dean had managed to gather up all the stray pills on the bed spread, so that now only two rested in his palm. He gestured for the glass and Sam shook his head. "You're shaking like crazy. Open up."

Dean looked at him petulantly but tossed the pills onto his tongue and let Sam press the glass of water against his lips. He tipped it and Dean drank the cool liquid down, swallowing the pills. He emptied the glass and Dean swatted his hand towards Sam.

Sam chuckled and set the glass down on the nightstand. "You're always so resistant to let me help you. It's okay, Dean."

Dean glared at his little brother before replying, "_Sam_. GO. QUESTION. THE. _WITNESSES_." Dean was practically snarling at this point, trying to impress his will upon the younger Winchester.

Sam had seen this time and again, and knew how it always went: If he _listened _to Dean, he'd be in the middle of questioning the first witness when Dean would call him, panicked. Sam would drive back quickly and Dean would convince him to stay until the next day… or the day after. So now he didn't waste the gas going out. All of the hard-hitting questions had to be worked up to, anyway… They never learned anything important in the first few minutes. It would be an exercise in futility.

"Dean, tell me: what do you need?"

He knew _exactly_ what Dean needed – this was just a test. He needed Dean to say it; needed him to be okay with it. Sure, he _had_ been okay with it for years now – ever since Sam was 16. But things had been changing, slowly, for the last few years. Changing because of Cas.

"Come on Sammy, don't make me _say _it…" Dean had wriggled out of his undershirt, his chest already shining with a thin sheen of sweat.

The smell was nearly overpowering by this point, but Sam managed to control himself. "You gotta say it, Dean. You know why."

"_Okay, okay_," Dean replied, his voice suddenly small. He looked up at Sam, his green eyes wide and pleading. "Sam please, I _need_ it. Give it to me."

"What do you need, Dean? Anything." He shrugged out of his suit jacket and tossed it on his bed, waiting for the other's reply.

"You _know_, Sammy… I need to be fucked. I need your knot."

Sam's breath hitched at Dean's reply. It always did. He couldn't help it. He breathed in Dean's heady scent, his nostrils flaring. "And why's that, Dean?"

"_Jesus, Sammy_. I'm. In. Heat. Just knot me!"

"God, you omegas are so good at begging for it…"

Dean glared up at his younger brother at the mild insult. "I changed my mind – I'll do it myself. Get the fuck out of here, you knot-head."

Sam started – this was new. Sure, he was used to his older brother calling him a "bitch" when he tried to cuddle or raising his hackles when his hormones weren't dominating him. But even though the _words_ weren't new, the resolve behind them was.

"Okay, Dean – whatever you want." Sam lowered his head; he wasn't going to pressure him. He knew it was hard enough for his brother to avoid jumping the closest agreeable alpha when he was in heat. If he was going so far as to turn him away, there had to be a reason.

Sam grabbed his jacket and stepped towards the door.

"Is this because of Cas?" Dean met his eyes, worry creasing his brow. "You know he's a beta, right?" Sam asked gently.

"_You don't know_ _that_," Dean bit back. His chest heaved, his lower lip trembling.

Sam raised his hands in surrender, and moved his hand to the doorknob when Dean said, "…Wait."

"Yeah, Dean? You need anything? I think I saw a sex shop outside – "

"Get over here, you idiot."

"You sure?" Sam's hand was still on the door knob, his gaze trained on his brother. Dean started unbuckling his pants.

As he pulled the zip, he replied, "Yeah… yeah, Sammy. Need you."

Sam walked over to Dean, gently caressing his cheek. "Sorry," Dean mumbled, and Sam leaned closer.

"It's okay," he responded, "I understand."

And the truth was, he did. Sure, an alpha's ruts weren't as debilitating as an omega's heats, but they were damn close. Sam would wake up in the middle of the night, covered in sweat and cursing the pounding headache that crept across his temples as soon as he opened his eyes. Every sound was louder, every color brighter, every taste richer, and every smell sharper.

The first time it happened, he was sixteen and he swore he'd been wounded during a hunt. He bolted out of his bed and shook Dean awake. Dean had opened his eyes and sniffed the air reflexively, seeking danger.

"Dean, I'm dying. Help me! If I've turned, dad'll _kill_ me!" He smelled the sharp scent of fear cloying to his brother, but underneath that… what _was_ that?

"Why would dad – " Dean started to ask sleepily.

"I got bit during the last hunt," he whispered. Sam's arms flailed as he ripped his shirt off. "Where is it?" he asked frantically, trying desperately to keep his voice down. He craned his neck, trying to get a look at his back. His arms stretched, his fingertips trying to find any punctures in his skin.

Dean's nostrils flared and he felt himself growing slick. He shook his head to clear it and Sam stepped closer. Dean tucked his nose under the covers in response to the proximity and Sam's eyes went wide. "No… _Please, Dean_. You can't be scared of me now… _You HAVE to hide me!_"

Dean let the cover fall to his lap and he cleared his throat: "Sam, you're not a vampire."

"But Dean – the smells, the lights!" Even the sound of his own voice was grating on his ears. For some reason though, Dean's wasn't.

"You – you're presenting."

"I'm… what? _Now?!"_

"Late bloomer, I guess," Dean mumbled, rubbing his hand over the back of his neck nervously.

"I… I thought I was a beta. I can't – if I present like you, dad'll get rid of _both_ of us."

Dean's eyes darkened at the memory, but he shook it from his thoughts to calm his little brother. "Well, uh, you don't gotta worry." Dean sniffed the air again. "You're an alpha."

Sam's eyes widened. He backed up towards his own bed, suddenly not trusting himself. "But the sounds, the smells..?"

"You're in rut," Dean answered matter-of-factly. He didn't admit to Sam how he'd studied alpha physiology when he was younger, _so _sure he'd be one. And when it turned out that he _wasn't… _well, that was the year Dean stopped reading textbooks.

Sam sat down on his bed, the look in his eyes hollow. Everything would be different now, and he knew it.

Dean got out of bed, and headed towards the bathroom. "I'm gonna, uh, get a shower. Make it easier on you."

Sam watched the door close and heard the shower start. As Dean washed with scent-blocking soap and shampoo, Sam found himself gravitating towards Dean's bed. As Dean lathered on scent-masking conditioner and shaved his pits and groin (if sex-ed had taught him _anything_, it was that those hairs trapped pheromones against the body), Sam tucked himself into Dean's sheets. He'd done it before, had slept in Dean's bed when he was spooked or worried. He knew, logically, that this was different. That getting into his brother's bed was the last thing he should be doing right now. But the scent of the omega pulled on him in a way he couldn't explain. At first, as he burrowed under the covers, the smell comforted him. But as he continued to breath it in, he felt himself harden. He blushed, willing it to go away.

Dean dried himself off and looked around for ways to stall. He put on his scent-blocking deodorant – which he used to wear only on hunts – and decided to shave. At twenty years old, he finally had more than peach fuzz. He used his dad's electric razor, and slapped scent-blocking aftershave on his cheeks and neck.

Meanwhile, Sam had wrapped himself in Dean's sheets, cocooning himself in the scent of the omega. Before he could stop himself, he was grinding against the mattress, his cheeks burning.

Dean brushed his teeth quickly, and decided it was time to face the music. He had expected Sam to be aggressive, to be trying to destroy the room, or even peeing on Dean's stuff (marking territory wasn't something alphas did _often_, but it occasionally happened during first ruts). What he _didn't_ expect to find was Sam humping his bed, gripping his sheets and bucking his (now-naked) ass up in the air.

His omega instincts had warned him not to startle an alpha in rut – especially if he didn't intend to mate with them. But as he watched the new alpha desperately try to reach completion, he wished that he'd done more to make his appearance known. He was about to lock himself back up into the bathroom until morning when Sam looked up. His cheeks were flush, his lips glistening, his hazel eyes wide and currently sparkling deep blue. _"Dean?"_ he asked, his voice low and wrecked.

"Lemme get dad. He'll know what to do," Dean pleaded as the younger alpha tightened his grip on the sheets, burrowing his head in the pillow. With a rough moan, Sam came. Dean's eyes widened, embarrassed to look on as Sam's shaggy hair clung to his neck with sweat, his lithe and slightly-muscular frame shuddering against his bed, his cheeks burning with shame as he pumped his release into the bedding. Dean looked away, ignoring the heat pooling in his gut as he pulled his pj bottoms back on.

When Sam came down, he was pliant and agreeable – the complete opposite of an alpha. He suddenly looked up from the bedding and whispered, "Please, Dean. Don't." He wanted to say more but he sputtered, the words caught on his tongue.

Dean had managed to move past his brother, and had his hand on the door handle. "Why? You scared because he's another alpha?" Dean challenged.

"No… Rut. _N – need_ you…" Sam looked down dejectedly as he spoke, appearing so unlike his newly-presented alpha self that Dean temporarily forgot. "Need you, Dean. Such a sweet omega…" Sam thrust against his ruined sheets, and curled in on himself, cursing his quickly-swelling erection. Without someone to knot, he'd keep getting off like this until he was raw and sore.

"You can't ask that. C'mon, I'm your _brother_," Dean reasoned, his freckles disappearing behind a fierce blush.

"Dean, please… I'll never ask you for anything else. I – I'm not sure I can survive this."

He knew in his heart that it was bullshit. He knew that no alpha had ever died because he couldn't tie. But he also knew his baby brother… his scared, shaky brother that deeply _needed_ this… And he knew he was going to hell, but he didn't know how to deny him.

"You can survive this, Sammy," Dean assured him. "I got you."

"Dean," he whimpered, reaching for the omega.

"This is wrong, Sammy. I can find you someone else, some horny omega chick –"

"No! I don't know them. I'm… I'm scared. You _know_, Dean. You've had sex…"

Dean knew he wasn't going to change Sam's mind, not when he was like this. He walked closer to Sam and sat down on the bed. He took a deep breath, realizing if he made his next move, he could never undo it.

"D'you want to knot me?"

Sam exhaled audibly in relief. "Yes… Please Dean. Need to knot… only trust you."

"Sammy… if we do this, _no one_ can _ever_ know. Got it?"

Sam nodded quickly, pressing up against his back. He breathed in against his neck. "You smell… wrong," Sam said, confused.

Dean turned around, and nudged Sam's head down. "I'm not in heat, so you have to open me up if you want in," he explained, ignoring Sam's earlier statement.

Sam's mouth was on Dean's hardening cock in the next second and his index finger worried against Dean's hole. He was already a little slick but as Sam worked his cock, more slick leaked out. Sam painted his finger with the fluid and pushed in. Dean gasped at the intrusion and Sam began to move the digit in and out. Dean felt himself getting wetter and he managed to gasp out, "C'mon Sammy, more."

Sam worked another finger inside of Dean's hole and Dean could feel himself opening up to Sam's ministrations. Sam pulled off of Dean's cock with a wet pop and pushed Dean onto his back with his other hand. He pushed a third finger into Dean's hole, taking in the heady scent of Dean's slick as he watched the digits disappear into his body. When he'd finally worked him open enough, Dean said, "Okay, Sammy. You can do it now."

He pulled his fingers out and bent his head down. He licked at his brother's puffy hole, tasting the slick on his tongue. Dean gave a surprised gasp at the sensation, but didn't push his head away. The pleasure won out over his embarrassment.

With shaking hands, Sam lined himself up. He took a deep breath and the gangly teen pushed his hard cock into Dean's slickened entrance. Dean's eyes went wide: he'd never had anything up there outside of a heat, and he'd never taken an alpha's cock in general. Dean was stricken by how different it felt - it was uncomfortable, but didn't hurt exactly.

"This feels so good, Dean… _thank you_." Dean looked up into Sam's eyes and saw gratitude hidden beneath the desire.

"S-sure," Dean responded, still adjusting to the feeling when Sam started moving. He tried to move slow but the urge to breed, to mate, was too strong and he soon found his hips pounding hard against Dean's ass.

"You gonna knot me, Sammy? Gonna cum inside me?" Dean urged as he stroked himself quickly. "It's okay baby, you can do it," he coaxed, feeling Sam's knot begin to swell. As it swelled further, catching on his rim, Sam slammed into Dean, his wiry arms holding Dean's hips down. With a final push, Sam's knot locked them together. He didn't seem to notice at first and kept trying to thrust until the movement hurt. As Sam started to cum inside Dean, Dean felt himself spurting his release all over his own stomach. Sam lay over Dean and moaned quietly, his cum continuing to spill into his older brother.

They lay there until Sam softened, and then uncoupled. Dean stroked Sam's bangs off of his forehead and smiled before stating, "Can we do that when I go into heat, too?"

Sam's broke out into a blissed out grin. "Definitely," he responded.

He broke out of his reverie and noticed Dean was tugging off the last of his clothes. "Come on Sammy, get in me," he begged, turning over onto his stomach.

Sam dropped down and licked the slick off of the inside of Dean's thighs. Dean gave a gasp of pleasure. Unlike the soft kitten licks Sam used when he was in rut, when Dean was in heat, his actions became obscene. He'd press his mouth right up to the entrance, tonguing him deeply to taste the honey-sweet slick. He'd usually get Dean off on just his tongue before probing in deep, his large fingers stroking his swollen prostate. He'd bring Dean right to the edge before pulling back so Dean could come down. As Sam did this now, Dean cursed into the crook of his arm, shielding his eyes from the brightness of even the dimmest lamp bulbs. "You know it hurts you to have more than three, and I already gave you one. If I let you now, you won't be able to take being knotted, and that's what you need."

He had had this particular conversation with Dean before, had given this explanation almost every heat the two had spent together. Saying it still hurt him, all the same.

"C'mon Sammy, do it already… I can't take any more teasing," Dean all but whined.

Sam nodded, and sat on the edge of the bed. He patted his lap and replied, "C'mon Dean, number five."

Numbering the positions had been Sam's idea. Dean was often so foggy during the first day of his heat that he could barely follow directions unless he was _already_ being fucked. So when they were both clear-headed, the two decided that this was easier. If there was one thing they could count on, it was that a sex-addled brain could remember ways to fuck.

Dean straddled Sam's lap, his ass pressed up against Sam's stomach. He grabbed Sam's thighs for balance and sunk onto Sam's cock, his thighs caging Sam's in. When Dean had finally taken it all (Sam was big, even for an alpha), he took a deep breath as Sam spread his own thighs wider. Dean felt his rim stretch tighter around the base as his cheeks parted and when he looked down, his cock jutted out from his body proudly. His balls, heavy with unspent seed, dangled in the empty air as Sam grabbed Dean by his hips and begin to move him up and down.

Although Dean was only a few inches shorter than Sam, in this position his feet dangled and he couldn't get any leverage. Sam gripped him tightly, the pressure nearly possessive and exactly what Dean needed. "There… Feels better right?"

Dean nodded, lost in the sensation of being filled by an alpha. Toys could get him through a heat, but his body always seemed to _know _somehow. His heats were longer and more miserable without a flesh-and-blood alpha in his bed. It didn't even have to be a male, but that made it much easier. He'd spent two heats with Lisa, and both had been… awkward. A female alpha, while a good mate for many male omegas, didn't seem to be right for Dean.

Dean felt Sam's cock brush his prostate and he let out a high moan. His eyes widened, the omega responses still vaguely shaming him. Sam made a pleased sound as he shifted his hips to elicit the noise again. When it escaped Dean's lips, Sam reached one hand forward to brush his thumb over Dean's slit. As he smeared the bead of precum over the tip, he brought the thumb to his lips. His tongue darted out and licked it, uttering a rough moan as he savored the taste. Sam's hand quickly closed over Dean's hip and he resumed his pace.

The previous action, combined with the increased pace, had Dean sobbing out moans of "_So_ _good_," and _"Fuck, Sammy…"_

As Sam's lips ghosted over the back of his neck, Dean heard an unmistakable flap of wings, and in the next second, Cas was standing two feet in front of them. "Dean, there's a case in Montana and –" The sentence died on Cas' lips as he took in the sight before him: Sam was perched near the foot of the bed, his lips pressed to Dean's neck and his long hair damp with sweat. His spread legs spreading Dean's even wider balanced a naked Dean between them.

Dean's face was flushed with arousal; his pupils blown wide so only thin circles of green could be seen around them. His hand tangled in Sam's hair, while the other gripped his thigh tightly to resist the impulse to stroke himself. His chest and stomach heaved with exertion, the pink head of his cock leaking precum down the sizeable shaft. A pulse of want travelled through him at the sight.

Cas looked away, the faintest prickle of a blush staining his cheeks. "I… apologize. I tried calling and there's a poltergeist…"

Even through the fog of the heat, Dean panicked – or tried to, anyway. He moved both of his hands to Sam's legs and was about to shoot off of Sam's cock (and hopefully up into the ceiling to avoid more embarrassment) when Sam whispered, "Dean. _Stop._" He didn't register what Sam meant until he realized he couldn't move. Sometime between the time he'd last spoken and the second Cas stopped speaking, Sam had knotted him. His knot wasn't fully swollen but Dean had figured the added pressure was the way he'd involuntarily clenched when Cas showed up. Even on a good day, they'd be tied for 20 minutes.

"Cas," Dean started, immediately annoyed his voice wasn't stronger, that it came out breathy, "'m…in heat. Needed an alpha…" Dean looked up at Cas reluctantly, and Cas seemed to come to a slow understanding.

He was quiet for a few minutes before he spoke again. Cas looked down at himself, at his quickly-growing erection, and met Dean's eyes. "You're an omega?" he asked in amazement. Dean, if possible, flushed even redder as he nodded.

Dean vaguely registered the full press of Sam's knot pressing against his prostate and he squeezed his eyes tight, his second orgasm wracking his body as he felt the steady pulse of Sam's cum begin to fill him. His cock twitched, another trickle of precum leaking out of him as his body gently shook. Sam held onto him tight, his concentration waning from Cas and Dean's exchange as Dean clenched around him again.

He didn't know if he actually saw hurt in Cas' eyes or if he imagined it, but before he could control it he hear himself explaining, "I'm sorry, Cas. Sam's the only alpha I know and – "

"No he's not," Cas stated, and Dean expected admonishment. Something along the lines of _"Control_ _yourself,"_ or _"What's wrong with you? He's your _brother." But instead, what Cas said was, "I'm an alpha."

Sam felt Dean tighten around him again and was seriously starting to feel like a third wheel. If he wasn't physically stuck in his brother right now, he'd slam the door so these two idiots could make moon eyes at each other in private.

Cas continued, "My vessel, Jimmy, took suppressants. By the time his spirit had left, I knew the routine. And I was given some sort of shot – a form of birth control, I think, the last time Jimmy had control. I believe another shot is required to reverse it." Dean's heart skipped a beat – _there_ _was a_ _reason._

He stepped closer, lessening the distance between him and Dean to a foot. "I'm reacting to you. Dean, you smell wonderful."

"I – I'm in heat for a week…" Dean began hopefully.

"Dean…" Cas replied, licking his lips, "Can I?" He left the question vague, gave Dean the chance to change his mind. The alpha in him was barely suppressed by his grace, and prodded at him to just take Dean. Sam hadn't staked a claim. Dean was an unbonded omega.

Sam's orgasm had ended minutes ago, but knew he wouldn't deflate for another ten minutes. He huffed hot breath out against the back of Dean's neck, and wrapped a hand around Dean's cock. He began to stroke gently, bringing Dean back to their current predicament. Dean shuddered under the touch, and looked up at Cas.

"I need to cum," he responded, bucking his hips as much as he could into Sam's hand. His eyes flicked down to Cas' groin, his cock straining against the slacks, and back up into his eyes. Cas stepped closer and dropped to his knees. Without another word, he wrapped a hand around the base of Dean's cock, his other hand gently moving Sam's. Sam curled both hands around Dean's hips as Cas took Dean's dripping cock into his mouth.

When Dean had allowed himself to imagine this, he thought Cas would be shy. That he was a clueless little beta who would love to choke on his cock and be fucked by him. But Cas, alpha Cas, was eager, his tongue lapping at the base in no time. As he bobbed his head up and down, his blue eyes made contact with Dean's. Dean's spine snapped tight and he came down his throat, a moan that was a garbled mixture of "Sam" and "Cas" leaving his lips. As he came, he tightened around Sam's cock one last time. Sam let out a low groan at the feeling, his oversensitive cock barely able to take the stimulation.

When Dean finally stopped jerking, Cas drew off. The taste of the omega inflamed his alpha instincts and he had to concentrate on reining them in with his grace. Sam finally felt his knot begin to deflate and he pulled Dean off of him. Dean stood shakily as Sam sidled past him towards the bathroom. "You guys have a lot to talk about," Sam said quickly, "…I'll be in here."

Sam had never cut and run after sex before and it felt weird. But then again, he'd never had his partner have a full conversation with another person _while_ he knotted them, so he supposed there was a first time for everything. He got in the shower and reached for the scent-blocking soap. No need for any stupid alpha posturing now that he knew he wasn't the only game in town.

END.


End file.
